1. FIELD OF THE INVENTION
The present invention relates to a permanent magnet DC machine. More particularly, the present invention relates to a method of preparing a permanent magnet DC machine having a stator in which anisotropic permanent magnets are used as main magnetic poles and auxiliary magnetic poles having higher reversible permeability than that of the main magnetic poles are respectively arranged adjacent to the main magnetic poles.
2. DISCUSSION OF BACKGROUND
In conventional permanent magnet DC machines such as starters, auxiliary magnetic poles made of a material such as soft iron are arranged at a side of main magnetic poles to increase magnetic function of the main magnetic poles against demagnetization due to armature reaction as disclosed in, for instance, Japanese Examined Patent Publication Sho 48-35721. In more detail, a stator is provided with the main magnetic poles made of anisotropy permanent magnets whose both ends are cut along the lines extending in parallel to the center line radially extending from the center of a rotor, and the auxiliary magnetic poles of soft iron each adjoining each of the main magnetic poles with an air gas in the circumferential direction, or each of the auxiliary magnetic poles is entirely in contact with a side surface of each of the main magnetic poles.
However, as well known, the anisotropic permanent magnet allows a magnetic flux to pass in the radial direction. Accordingly, when the auxiliary magnetic poles are arranged in the circumferential direction (the direction of rotation) so as to be in close-contact with the main magnetic poles, leakage magnetic paths are formed between the main and auxiliary magnetic poles in a close-contacting state. Thus, in the permanent magnet DC machine with the main and auxiliary magnetic poles whose opposing faces are entirely in close-contact with each other reduces its an effective magnetic flux, whereby a rotating torque decreases.
On the other hand, in the permanent magnet DC machine with the main and auxiliary magnetic poles in which air gaps are formed between the adjacent main and auxiliary magnetic poles in the circumferential direction (the direction of rotation), there is found a steep depression in the waveform of magnetic flux distribution at the time of the application of a high load at the armature side of the rotor because the air gaps are formed in the circumferential direction over the entire length, even though the leakage magnetic path can be reduced. Accordingly, a smooth operation of the DC machine can not be expected. Further, the generation of magnetic sounds and vibrations take place at the same time of reduction of power. In addition, from the viewpoint of the construction, it is difficult to determine positions of the main and auxiliary magnetic poles because of the presence of air gaps, and accordingly effective assembling works can not be obtained.